1. Field of the Invention
The invention relates to an MR method for determining the nuclear magnetization distribution in an examination zone, in which sub-image data are acquired by means of a surface coil arrangement which comprises at least two surface coils and which has a locally inhomogeneous sensitivity, MR superposition images being formed from said sub-image data by weighted summing.
2. Description of the Related Art
From German Patent Application P 44 27 429.7-33 it is known to acquire sub-image data for a respective sub-image by means of a respective surface coil and to combine these sub-images so as to form an MR superposition image (or overall image) by weighted summing.
The determination of the nuclear magnetization distribution in an examination zone by means of a surface coil arrangement comprising at least two, but in practice usually more than two surface coils, offers the advantage that the signal-to-noise ratio is substantially better than in the case of coils capable of receiving the resonance signals from the entire examination zone. It is a drawback of this method, however, that artefacts which are present in some sub-images only are superposed on the artefact-free sub-images upon combination of all sub-images into the overall image, so that these artefacts appear in the overall image.
Artefacts, for example motion artefacts, are often caused by local movement of anatomic structures, for example the (bright) fat of the abdomen during respiration, the pulsation of large blood vessels, or the "fresh" magnetic spins flowing into (essentially stationary) vessels within the slice reproduced. In dependence on the frequency of the motion, this causes stripes in Fourier methods which are continuous or consist of periodic repetitions (ghosts) of the moving object and which extend through the pixels concerned across the image. These stripes also cover remote areas and individual "ghosts" may imitate pathologies (metastases) (for example, in the liver) or compensate their contrast (when a bright artefact coincides with a low-signal metastasis of the same size).